1. Field of the Invention
This invention relates to a light emitting device that a light emitting diode (hereinafter referred to as LED) as a light source is sealed with a moisture-impermeable material such as glass.
Also, this invention relates to a light emitting device that heat generated from a high-output light emitting element can be effectively radiated.
2. Description of the Related Art
Conventionally, light emitting devices using LED as a light source are known. An example thereof is a light emitting device that an LED element is integrally sealed with sealing resin such as epoxy resin.
The epoxy resin is widely available and is easy to mold. Therefore, it is widely used as a sealing material for light emitting device. However, the epoxy resin is gradually colored and turns yellow when it is irradiated by intensive light emitted from the LED element. If it is thus colored, the light output of the light emitting device must be reduced because light emitted from the LED element is absorbed by the colored portion.
Japanese patent application laid-open No. 11-204838 (herein referred to as prior art 1) discloses a light emitting device that an LED element is sealed with a moisture-impermeable glass layer with phosphors contained therein.
FIG. 1 is a cross sectional view showing the light emitting device disclosed in prior art 1. The light emitting device 50 is composed of: wiring conductors 51, 52; a cup portion 53 formed in the wiring conductor 52; the LED element 54 bonded at a bottom 53A of the cup portion 53; wires 55, 55 that each electrically connect between the electrode of LED element and the wiring conductors 51, 52; a glass layer 56 that seals the LED element 54 disposed inside the cup portion 53; phosphors 56A contained in the glass layer 56; a lamp-shaped sealing resin 57 that is transparent and seals all the above elements.
In this structure, since the LED element 54 is sealed with the glass layer 56 filled in the cup portion 53, the reduction of light output due to colored sealing material can be avoided. Further, the degradation of phosphor can be prevented because the glass layer blocks the permeation of moisture.
However, the light emitting device of prior art 1 has the next problems:
(1) Since the viscosity of glass layer is significantly greater than that of resin, air bubbles must be contained in the glass layer in sealing the LED element with the glass material. The air bubbles once contained therein are difficult to separate. Therefore, the air bubbles will remain in the glass layer.
(2) In sealing the LED element with glass, it is processed at a high temperature of 300° C. or higher. After the processing, the residual stress in the glass material is generated due to a difference in thermal expansion coefficient between the LED element and the glass material. The residual stress may cause a crack in the glass material when being subjected to a heat shock.
In recent years, high-output LED's are developed and a large output type LED at several watts output level is already manufactured. Although LED's are characterized by small heat release, the high-output (high-brightness) LED element must generate a considerable amount of heat because of a large current flow passed therethrough.
Japanese patent application laid-open No. 2000-150967 (herein referred to as prior art 2) discloses an LED package that a slug as heat sink is disposed below an LED element (LED die).
In the LED package of prior art 2, the slug is inserted to a lead frame over-molded with an embedded plastic member. The LED die is attached directly or indirectly to the slug through a sub-base that has a good thermal conductivity.
In this structure, since the LED die is thermally connected to the slug, the LED die can be kept at a bonding temperature lower than that of conventional packages. Because of being at the lower temperature, the LED die may be not subjected to a large thermal stress and, therefore, reliability and good characteristics thereof can be secured even at large power operation.
However, the LED package of prior art 2 has the next problems:
(1) The secondary support member (embedded plastic member) to over-mold the lead frame is required. Therefore, the number of parts must be increased and it is difficult to provide a compact package.
(2) Further, since the secondary support member is made of resin, the LED package does not have such a high heat resistance that it can be applied to the lead-free reflowing.